IgG dimers have been previously characterized as idiotype (Id)-anti- idiotype pairs. The percentage of dimer in IgG preparations increases as a function of the plasma pool size (i.e., number of donors). Experimentally, for value of N between 20 and 10,000, % dimer is approximately equal to -9.14+8.32xlog N, where N is the number of donors. A possible mechanism for this phenomena is that as the antibody diversity increases in a pool, it becomes more probable that a particular antibody and its anti-Id will both be included. A model has been developed to explore the relationship of dimer content to pool size. This model assumes the following: (1) the Id repertoire of the species is finite; (2) for each possible Id there exists one and only one alit-Id; (3) Id and alit-Id will form a dimeric pair with high affinity; and (4) the number of different Ids produced by an individual at a given time (Ri) is smaller than the total number of different Ids (Rt) capable of being produced by the human species and the former is selected from the latter in a random manner. This model predicts that nor N less than 0.1xRt/Ri, % dimer is approximately proportional to N; for values of N between 0.1xRt/Ri and 10xRt/Ri, % dimer is approximately proportional to log N. As N increases still further, the % dimer asymptotically approaches 100%. Thus, the model successfully predicts the semilogarithmic relationship between % dimer and N that is experimentally observed. In the semilogarithmic portion of the curve, the model predicts that % dimer will increase by 40.9% for a tenfold increase in N, whereas the experimental data demonstrates an increase in dimer of only 8.32% for a tenfold increase in N. One possible explanation for this discrepancy is that the selection of Id repertoire by an individual is not entirely random, but rather tends to be "similar" from one individual to another. Finally, a comparison of the experimental data with the theoretical model suggests that Rt/Ri is approximately equal to 180, i.e., the species idiotypic repertoire is about 180 times larger than that of an individual at a given time.